The ability to form nonwoven fabrics from batts of random, loose thermoplastic fibers and the like has been recognized in the prior patent literature. For instance, see German Offenlegungsschrift No. 2,259,203 and U.S. Pat. Nos. 3,949,127 and 3,966,519. U.S. Pat. No. 4,086,112 teaches the direct printing of heavyweight fabrics by feeding the fabric and heat transfer print paper between a horn and an anvil having large bonding areas. Prior art methods of production of nonwovens ultrasonically have experienced a number of drawbacks, however, when applied to light-weight batts, which have adversely affected the widespread commercialization of the ultrasonic production of nonwovens.
Prior art techniques for the ultrasonic formation of nonwoven fabrics from lightweight (under about 3 oz./yd..sup.2) thermoplastic batts have encountered one or more of the following drawbacks: Unacceptable fabric properties (especially strength and hand). Slow processing speeds. Equipment malfunction due to burnup of the anvil roller should the batt break or cease to be fed between the horn and roller, or should scrap metal inadvertently appear in the batt. Unacceptable aesthetic characteristics of the fabric, especially surface characteristics.
According to the present invention, all of the above drawbacks are overcome, and additionally it is possible to produce printed and decorative fabrics directly from lightweight fiber batts without a dyeing step.
According to the method of the present invention, a lightweight nonwoven fabric is formed from a batt of random, loose, ultrasonically fusible (i.e., thermoplastic) fibers, utilizing an ultrasonic welding machine having a patterned roll anvil or the equivalent, and a horn, and a carrier of flexible sheet material capable of drawing and spreading out ultrasonic energy from the machine. The method comprises the steps of disposing the lightweight batt and carrier in face to face abutting relationship; progressively automatically feeding the batt and carrier together between the horn and anvil of the machine; supplying ultrasonic energy to the batt with the machine sufficient to form the batt into a nonwoven fabric; and separating the batt and carrier from each other. The use of the carrier unexpectedly results in increased fabric strength for lightweight fabrics (i.e., 3 oz./yd..sup.2 and less, depending on fiber denier), results in a fabric having smooth surfaces and good textile hand, and thus better aesthetic characteristics, allows a quantum increase in speed of bonding and positively prevents damage to the anvil roller even if the batts being fed to the ultrasonic machine should break or scrap metal should inadvertently become associated therewith.
In practicing the present method, the carrier may be disposed against the horn and the batt against the anvil or vice versa. Normally, the lightweight fabric may be formed more quickly if the carrier is disposed adjacent the horn, however, for formation of some specialized fabrics, the carrier should be disposed against the anvil and for a smooth surface fabric on both faces the carrier is more preferably disposed against the anvil.
One improved fabric that may be produced according to the present invention is a decorative or printed lightweight fabric with good textile hand which may be produced without any accessory forming steps (except heat setting), such as dyeing. In forming a decorative fabric, contrasting fibers--acrylic fibers--are disposed on the batt either randomly or in a given pattern by a yarn control machine prior to feeding the batt through the ultrasonic welding machine. The contrasting fibers become part of the nonwoven fabric formed. Printed fabrics are formed by using heat transfer print paper as the carrier, and then passing the formed lightweight nonwoven fabric and transfer paper through a heat setting operation before separating the carrier from the formed fabric. The quality of the print on the formed fabric utilizing this technique is substantially identical to the print quality when heat transfer print paper is used in conventional techniques.
According to the present invention, it has been recognized that the source of the problem of insufficient fabric strength or hand characteristics that resulted in the prior art in bonding lightweight fabrics, was the size and spacing of contact points. The patterned roll (or equivalent) that is utilized must have points of optimum size, and an optimum number of points per unit area. If the weld points are too large, the fabric produced will be weaker and the fabric is unacceptable from an aesthetic point of view having poor hand and surface characteristics; if the weld points are too small the fabric will also have less strength than desired. According to the invention, the points of the patterned roller or the like that come into contact with the fabric have an area of about 0.003 sq. in., which has been found to be the optimum for fabric strength and textile hand.
If further fabric strength is required, Heterofil binder fibers (or like bicomponent core and sheath filament fibers) may be utilized, typically in blends of 25% and 50%, with polyester or other thermoplastic fibers.
Suitable carriers include brown paper, tissue paper, heat transfer print paper, Nomex, nonthermoplastic sheets in general, and rayon. In general, the carrier may be any flexible smooth-surfaced sheet material that is readily detachable from the formed fabric (that is, will not be ultrasonically bonded irremovably to the formed fabric), and is capable of drawing and spreading out ultrasonic energy from the machine.
It is the primary object of the present invention to produce improved lightweight nonwoven fabrics from lightweight fiber batts utilizing ultrasonic bonding, with increased process efficiency. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.